The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery. Such screws have a head that can swivel about a shank of the bone screw, allowing the head to be positioned in any of a number of angular configurations relative to the shank.
Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate rods are often utilized that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury or disease. Such rods must be supported by certain vertebrae and support other vertebrae.
The most common mechanism for providing vertebral support is to implant bone screws into certain bones which then in turn support a rod or are supported thereby. Bone screws of this type may have a fixed head relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do. Therefore, polyaxial bone screws are commonly preferred.
Polyaxial bone screws allow rotation of the head about the shank until a desired rotational position of the head is achieved relative to the shank. Thereafter, a rod can be inserted into the head and eventually the head is locked or fixed in a particular position relative to the shank.
A variety of polyaxial or swivel-head bone screw assemblies are available. One type of bone screw assembly includes an open head that allows for ease in placement of a rod within the head. A closure top or plug is then used to capture the rod in the head of the screw.
Because such implants are for placement within the human body, it is desirable for the implant to have as little effect on the body as possible. Consequently, heavy, bulky implants are undesirable and lighter implants with a relatively small profile both in height and width are more desirable. However, a drawback to smaller, lighter implants is that they may be more difficult to rigidly fix to each other and into a desired position. Lack of bulk may also mean lack of strength, resulting in slippage under high loading.
A drawback of some implants is that in an effort to provide an open, swivel head screw that is easy to assemble, the interior of the bone screw head is largely hollowed out, resulting in a cavity that may provide adequate space for the attachment of component parts within the cavity, but also may result in a head of reduced mass and thus possibly of reduced strength. If the head cavity size is reduced to maintain adequate bulk and strength of the head, the other bone screw components utilized to attach the shank to the head must be reduced in size or otherwise reconfigured, potentially resulting in bone screw shank components of lesser strength. Another potential difficulty is lack of working space for utilizing manipulating or fastening tools to attach the component parts to one another.
Furthermore, certain part configurations and/or reduced sizing of the component parts may result in limitations on the frictional, load bearing surfaces within the screw head once the screw is implanted on a vertebra and the head is fixed in a set angle with respect to the shank body. For example, a select portion of the interior of the head may be removed to create a recess so that other component parts of adequate bulk may be inserted in the head. This may result in excessive local pressure on other surfaces of the head after shank installation, which in turn may lead to localized damage and eventual loosening or deformation of the component parts. It is therefore desirable to draw a balance between mass and strength of the individual bone screw component parts, number of component parts, the configuration thereof, and ease of installation of the implant component parts to each other and to the vertebrae.